# Standard library header <cctype>

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< cpp‎ | header

This header was originally in the C standard library as `<ctype.h>`.

This header is part of the null-terminated byte strings library.

### Functions
isalnum | checks if a character is alphanumeric (function)
isalpha | checks if a character is alphabetic (function)
islower | checks if a character is lowercase (function)
isupper | checks if a character is an uppercase character (function)
isdigit | checks if a character is a digit (function)
isxdigit | checks if a character is a hexadecimal character (function)
iscntrl | checks if a character is a control character (function)
isgraph | checks if a character is a graphical character (function)
isspace | checks if a character is a space character (function)
isblank (C++11) | checks if a character is a blank character (function)
isprint | checks if a character is a printing character (function)
ispunct | checks if a character is a punctuation character (function)
tolower | converts a character to lowercase (function)
toupper | converts a character to uppercase (function)

### Synopsis

    namespace std {
      int isalnum(int c);
      int isalpha(int c);
      int isblank(int c);
      int iscntrl(int c);
      int isdigit(int c);
      int isgraph(int c);
      int islower(int c);
      int isprint(int c);
      int ispunct(int c);
      int isspace(int c);
      int isupper(int c);
      int isxdigit(int c);
      int tolower(int c);
      int toupper(int c);
    }

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